The prior art within borehole logging and data acquisition is based, to a great extent, on photomultiplier tubes or photodiodes connected to scintillator crystals such as potassium iodide or caesium iodide.
When an assembly of a scintillator crystal connected to a photomultiplier tube is exposed to ionizing radiation (such as X-, gamma or particle radiation), the incident radiation will be converted into non-ionizing, “optical” photons in the scintillator crystal through a process which includes scattering, nuclear recoil and/or fluorescence. The optical photons are then detected, that is to say counted, by means of the photomultiplier tube which is connected to the scintillator crystal. As mentioned above, the multiplier tube may be substituted with a photodiode for the same purpose.
A typical borehole application for such assemblies comprises borehole logging. In such assemblies it is desirable that the ionizing radiation should be as large as possible for the collection of photons to be the largest possible with a view to improving the statistical analyses of the data acquired and thereby reducing errors in the readings. For that reason and because of the cylindrical form that most tools for use in boreholes have, such a detector is typically formed as a cylindrical scintillator with a photomultiplier or a photodiode connected to one end of the scintillator. The concept consists in maximizing the collection in a unit volume of photons moving radially in towards the tool in a direction perpendicular to the longitudinal axis of the borehole. Even though scintillators are in general use, the scintillator has physical properties that do not make it well suited for maximum collection of inflowing photons. When an inflowing ionizing particle or a photon interacts with the scintillator material, the result is a yield of scintillated photons of less energy and with a resultant direction statistically distributed around the point of interaction, that is to say that the direction of the outgoing, optical photon is generally different from the direction of the incident photon, depending on the specific interaction between the photon/particle and the atoms in the scintillator. Based on this, it is obvious that, statistically, scintillated or optical photons appear in all directions from the scintillator, independently of the direction of the incident photons or ionizing particles. Since the photomultiplier or photodiode is attached to one end of the scintillator, the maximum detectability of the apparatus is restricted to the portion of optical photons entering the photomultiplier tube or the photodiode. Based on the fact that the surface of a cylinder is described as 2πr2+2πrh, r being the radius of the cylinder and h being the height, the portion of optical photons reaching the photomultiplier tube or photodiode, is expressed as πr2/(2πr2+2πrh), resulting in just 33% detection for a scintillator cylinder with h=r, or 25% detection for a scintillator in which h=2r, or 14% for h=3r. The detection rate reaches 100% only when the cylinder height is set to zero. An is obvious solution to this problem is to place a photomultiplier tube or a photodiode at both ends of the scintillator cylinder. Even though this has the effect of doubling the efficiency, the collection efficiency will stay way below 100%.